1. Field of the Invention
The invention relates generally to sublimators used for heat rejection.
2. Background Art
During manned space missions, it is important to control the environment for the well being of the human participants. Paramount among environmental concerns is the dissipation of heat that may accumulate from the combined metabolic heat given off by the passengers and waste heat from electronics. One strategy that has been used to dissipate heat is evaporative cooling or evaporative heat rejection. Several designs have been developed that use sublimators as a means of dissipating unwanted heat for vehicular and space suit cooling.
A sublimator is an evaporative heat rejection device that provides cooling by evaporative venting of water vapor into space, transferring the latent energy in the water vapor away from the vehicle. Water has a latent heat of vaporization of 2461 kJ/kg, which makes evaporative cooling an effective process for dissipating unwanted heat. These devices take advantage of the vacuum of space and the phase properties of water below its triple point temperature to remove water vapor directly from the solid phase (ice) by a process called sublimation. This is possible because below the triple point pressure of water (4.56 mmHg), water exists either in solid phase (ice) or gas phase (water vapor) depending on the temperature.
Typically, the sublimation process in a sublimator device begins by delivery of feed water to a porous substrate surface with one face exposed to a vacuum. The low pressure causes the water vapor to freeze within the pores of the substrate. Eventually, a layer of ice forms filling the substrate pores. Delivery of heat, via a heated coolant, to the porous substrate causes sublimation of the ice. Water vapor is vented into space with the net effect of the dissipation of heat. The cycle starts anew as more feed water replenishes the ice layer in the porous substrate. Most importantly, the process is self regulating because the water flow rates are controlled by the ice layer. Although this example shows the use of water as the evaporant (sublimant), other evaporants may be used such as R134a. However, a layer of ice would not form if a refrigerant was used and the evaporant flow rate may require additional controls.
Sublimators known in the art control the evaporant flow rate by use of a single porous substrate with a precise range of pore sizes. Pores that are too large cause the rapid loss of evaporant. Typical porous materials may have pore sizes ranging in size from 3-6 μm. When the water sublimes from the porous substrate, non-volatile contaminants are often left behind in the small pores. Over time, the performance of the sublimator may be compromised by the accumulation of these non-volatile contaminants and the porous substrate requires replacement or removal and cleaning, both of which may be costly.
One solution to the accumulation of non-volatile contaminants is to separate the evaporant flow control element and the site of sublimation. If the sublimation portion of the device can be constructed such that it has very large pores, then it may be insensitive to the accumulation of non-volatile materials. Such a strategy has been disclosed, for example, by Curtis U.S. Pat. No. 3,613,775 in which a Teflon® (Teflon® is a registered trademark of DuPont, Wilmington, Del.) felt material is used to distribute feed water on to a metal surface covered with an open-cell foam with large pore sizes. It has been observed by those skilled in the art, however, that the Teflon® felt layer compresses over time, which results in a loss of efficiency in feed water distribution.